In order to purify the NOx, etc. contained in the exhaust gases emitted from automobile engines, engines for construction machines, industrial stationary engines, burners, etc. or to adsorb the hydrocarbons contained therein, there have been used catalysts produced by loading an ion-exchanged zeolite on a honeycomb-shaped ceramic carrier (a honeycomb structure) composed of cordierite or the like.
When a zeolite has been loaded on a ceramic carrier composed of cordierite or the like, the cordierite of the like shows no action such as NOx purification, hydrocarbons adsorption, or the like; consequently, the presence of the cordierite or the like causes an increase in pressure loss when an exhaust gas passes.
Meanwhile, there were proposed methods for producing a honeycomb structure per se by forming a forming material containing a zeolite ion-exchanged with a metal ion and then firing the formed material (see, for example, Patent Documents 1 to 3).    Patent Document 1: JP-A-2007-296521    Patent Document 2: U.S. Patent Application Laid-Open No. 2008-167178    Patent Document 3: U.S. Pat. No. 6,555,492
In the conventional methods for producing a honeycomb structure per se by forming a zeolite ion-exchanged with a metal ion and then firing the formed ion-exchanged zeolite, the ion-exchanged zeolite is exposed to a binder or high-temperature steam in the forming and subsequent firing, which has caused the movement and removal of metal ion in zeolite, the destruction of zeolite structure and the resultant reduction in catalytic activity and adsorption capability.
Further, in treating an exhaust gas using a conventional honeycomb structure composed of a zeolite ion-exchanged with a metal ion, i.e. a honeycomb-shaped zeolite structure, a major portion of the exhaust gas was treated around the surface of each partition wall and the amount of the exhaust gas penetrating into the inner portion of the partition wall was relatively small. Further, in the conventional honeycomb structure composed of a zeolite ion-exchanged with a metal ion, the distribution of the metal ion in the thickness direction of the partition wall was uniform, which made it impossible to effectively utilize the metal ion present in the inner portion of the partition wall.